go/ast: fix doc string for ast.GenDecl

Fixes #18109.

Change-Id: I5e3a44422794b7bae7741523fb7cacb6ba147af7
Reviewed-on: https://go-review.googlesource.com/33669
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: griesemer

src/go/ast/ast.go

@@ -902,7 +902,7 @@ type (
 
 	// A GenDecl node (generic declaration node) represents an import,
 	// constant, type or variable declaration. A valid Lparen position
-	// (Lparen.Line > 0) indicates a parenthesized declaration.
+	// (Lparen.IsValid()) indicates a parenthesized declaration.
 	//
 	// Relationship between Tok value and Specs element type:
 	//